The type of vibrating screens having a particular interest in the present invention is small in size and is normally used by landscape contractors, agricultural facility personnel and small vendors of sand, aggregate and top soil. An example of a vibrating screen of this type is described in U.S. Pat. No. 4,197,194 issued on Apr. 8, 1980 to James L. Read. This portable vibrating screen comprises a box-like frame having a tall end and a short end joined by sides, the short end being closed and the tall end being open. A shaker screen slopes downwardly from the tall end to an upper edge of the short end. The tall end of the frame is open to permit a pay loader to collect the finer material from within the frame.
A common problem with vibrating screens of this type is that the vibration of the screening basket is transmitted to the frame, causing the whole machine to shake and shift on the ground when operating. This shifting of the machine is often referred to in the trade as: "walking". Consequently, users of the machine often add weight to the lower part of the structure by partly burying it with gravel, rocks or cement blocks. However, this additional mass changes the stiffness of the frame and often increases the occurrence of fatigue cracking.
A vibrating screen is by its nature a nonlinear system where the mass carried by the screening basket is continuously changing. Therefore, the equations for finding the natural frequencies or resonance of the structural members, the characteristics of spring isolators and the sizes of mass dampers for such system are nonlinear. Hence, an ideal structure has always been difficult to determine with accuracy, and portable vibrating screens are generally known for requiring frequent tendance by a welder.